The present invention relates to an improvement in an intake passage structure for internal combustion engines, and more particularly to an intake passage structure for internal combustion engines which includes a primary air-fuel mixture passage designed for preventing vapor lock from occuring in a secondary slow fuel supply passage.
Internal combustion engines having a two-barrel carburetor for use on automobiles include a secondary slow fuel supply passage extending as a bypass passage from the carburetor to a secondary air-fuel mixture passage adjacent to a secondary throttle valve for supplying additional fuel into engine combustion chambers for improved transient engine operation when the secondary air-fuel mixture passage comes into operation in addition to the primary air-fuel mixture passage. Heat from the engine however has a tendency to vaporize fuel in the secondary slow fuel supply passage when the engine operates under a low load with only the primary air-fuel mixture passage being active. Such fuel vaporization causes vapor lock in the secondary slow fuel supply passage, which then fails to supply fuel when the secondary throttle valve is opened. Such a condition leads to engine operation irregularities such as hesitation and stumbling, and drivability of the automobile becomes worse. Known arrangements for preventing such difficulties include means for introducing a portion of fuel from the secondary slow fuel supply passage into the primary air-fuel mixture passage or an air induction pipe, and an adjustment for discharge characteristics of an accleration pump for the secondary slow fuel supply passage. However, these prior proposals are disadvantageous in that the former arrangement makes it difficult to control a small amount of fuel during idling, and the latter means has limitations on the possible range of adjustment and frequently fails to adjust the acceleration pump in a desired condition.